On the same day, major donors in Tokyo pledged $16 billion in development aid for Afghanistan over the next four years as they try to prevent it from sliding back into chaos once most foreign troops have left by the end of 2014.

The Taliban said early on Sunday that a roadside bomb had killed four American soldiers in eastern Logar province, where Czech soldiers are based. The exact location of the six dead NATO soldiers in the east was not immediately clear.

Three bombs hit three vehicles in Kandahar, the birthplace of the Taliban where the group has substantial sway and enjoys popular support, killing 18 people including children.

"Villagers were travelling in a mini-van and a tractor when they were hit by twin roadside bombs planted by the Taliban," the provincial governor's spokesman, Ahmad Faisal, said of the attack in Spin Boldak near the border with Pakistan.

A third bomb then killed a family of four in Arghistan district, also straddling the Pakistan border, local officials said.

Two policemen were killed by a bomb to the west of Kandahar in southern Helmand province, where clashes with militants killed a further four officers, the Helmand media office said.

Roadside bombs are by far the deadliest weapon deployed by Taliban insurgents in the war against NATO and the government.

Civilians bear the brunt of the violence. Although the UN reported a 20 percent decrease in civilian deaths in the first four months of this year from the same period in 2011, last year saw the number of civilians killed in Afghanistan rise for a fifth straight year to over 3,000.

NATO says the vast majority of these deaths are caused by insurgents, not by the coalition.